1. Field of the Invention
This invention relates to ethylene copolymers and more particularly, it relates to the manufacture of reduced melt index, low gel content ethylene copolymers.
2. Description of the Prior Art
Gel (insoluble polymer particles) has long been recognized as an undesirable contaminant in ethylene copolymers produced in free radical-initiated high pressure polymerizations. In appearance it varies from small particles, called "micro" gel or "background" gel to relatively large particles called "fisheye" or "lens" gel.
Gel is undesirably overpolymerized (very high molecular weight), or crosslinked, polymer not soluble in the bulk of the product at the temperatures which are normally encountered in processing and using the polymer. Gel detracts from the appearance of films prepared from essentially 100% polymer systems. It also affects adversely the appearance and ease of application of hot melt and solvent based formulations containing ethylene copolymers when these formulations are used to coat packaging substrates such as paper, paperboard, and corrugated board as well as finishing furniture, etc. Gel present in the polymer also appears to serve as the nucleus for the development of larger gel particles when ethylene copolymer-based formulations are held molten during blend preparation and application.
A number of solutions have been suggested to at least partially overcome these disadvantages. Polymer can be treated in high shear mixers to break down the gel. Polymer can be forced through fine mesh screens to filter out gel and/or break the gel into smaller particles so that it becomes less harmful. It is stated in U.S. Pat. No. 3,157,628 that, in a high-pressure tubular reactor, a small amount (0.05 to 0.20% based on the weight of the polyethylene produced) of an A-stage paratertiaryalkylphenol-formaldehyde resin can be injected into the last half of the elongated reaction zone to decrease the number of fisheyes and lenses present in films made from the polymer. The above patent further states that many products, such as di-tertiarybutyl-para-cresol (BHT), which are known to be free radical traps at lower temperatures, are not effective under the conditions covered by the patent.